/-
Copyright (c) 2021 Yury Kudryashov. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Yury Kudryashov
-/
import topology.urysohns_lemma
import topology.continuous_function.bounded

/-!
# Metrizability of a T₃ topological space with second countable topology

In this file we define metrizable topological spaces, i.e., topological spaces for which there
exists a metric space structure that generates the same topology.

We also show that a T₃ topological space with second countable topology `X` is metrizable.

First we prove that `X` can be embedded into `l^∞`, then use this embedding to pull back the metric
space structure.
-/

open set filter metric
open_locale bounded_continuous_function filter topological_space

namespace topological_space

variables {ι X Y : Type*} {π : ι → Type*} [topological_space X] [topological_space Y]
  [finite ι] [Π i, topological_space (π i)]

/-- A topological space is *pseudo metrizable* if there exists a pseudo metric space structure
compatible with the topology. To endow such a space with a compatible distance, use
`letI : pseudo_metric_space X := topological_space.pseudo_metrizable_space_pseudo_metric X`. -/
class pseudo_metrizable_space (X : Type*) [t : topological_space X] : Prop :=
(exists_pseudo_metric : ∃ (m : pseudo_metric_space X), m.to_uniform_space.to_topological_space = t)

@[priority 100]
instance _root_.pseudo_metric_space.to_pseudo_metrizable_space {X : Type*}
  [m : pseudo_metric_space X] :
  pseudo_metrizable_space X :=
⟨⟨m, rfl⟩⟩

/-- Construct on a metrizable space a metric compatible with the topology. -/
noncomputable def pseudo_metrizable_space_pseudo_metric
  (X : Type*) [topological_space X] [h : pseudo_metrizable_space X] :
  pseudo_metric_space X :=
h.exists_pseudo_metric.some.replace_topology h.exists_pseudo_metric.some_spec.symm

instance pseudo_metrizable_space_prod [pseudo_metrizable_space X] [pseudo_metrizable_space Y] :
  pseudo_metrizable_space (X × Y) :=
begin
  letI : pseudo_metric_space X := pseudo_metrizable_space_pseudo_metric X,
  letI : pseudo_metric_space Y := pseudo_metrizable_space_pseudo_metric Y,
  apply_instance
end

/-- Given an inducing map of a topological space into a pseudo metrizable space, the source space
is also pseudo metrizable. -/
lemma _root_.inducing.pseudo_metrizable_space [pseudo_metrizable_space Y] {f : X → Y}
  (hf : inducing f) :
  pseudo_metrizable_space X :=
begin
  letI : pseudo_metric_space Y := pseudo_metrizable_space_pseudo_metric Y,
  exact ⟨⟨hf.comap_pseudo_metric_space, rfl⟩⟩
end

instance pseudo_metrizable_space.subtype [pseudo_metrizable_space X]
  (s : set X) : pseudo_metrizable_space s :=
inducing_coe.pseudo_metrizable_space

instance pseudo_metrizable_space_pi [Π i, pseudo_metrizable_space (π i)] :
  pseudo_metrizable_space (Π i, π i) :=
by { casesI nonempty_fintype ι, letI := λ i, pseudo_metrizable_space_pseudo_metric (π i),
  apply_instance }

/-- A topological space is metrizable if there exists a metric space structure compatible with the
topology. To endow such a space with a compatible distance, use
`letI : metric_space X := topological_space.metrizable_space_metric X` -/
class metrizable_space (X : Type*) [t : topological_space X] : Prop :=
(exists_metric : ∃ (m : metric_space X), m.to_uniform_space.to_topological_space = t)

@[priority 100]
instance _root_.metric_space.to_metrizable_space {X : Type*} [m : metric_space X] :
  metrizable_space X :=
⟨⟨m, rfl⟩⟩

@[priority 100]
instance metrizable_space.to_pseudo_metrizable_space [h : metrizable_space X] :
  pseudo_metrizable_space X :=
⟨let ⟨m, hm⟩ := h.1 in ⟨m.to_pseudo_metric_space, hm⟩⟩

/-- Construct on a metrizable space a metric compatible with the topology. -/
noncomputable def metrizable_space_metric (X : Type*) [topological_space X]
  [h : metrizable_space X] :
  metric_space X :=
h.exists_metric.some.replace_topology h.exists_metric.some_spec.symm

@[priority 100]
instance t2_space_of_metrizable_space [metrizable_space X] : t2_space X :=
by { letI : metric_space X := metrizable_space_metric X, apply_instance }

instance metrizable_space_prod [metrizable_space X] [metrizable_space Y] :
  metrizable_space (X × Y) :=
begin
  letI : metric_space X := metrizable_space_metric X,
  letI : metric_space Y := metrizable_space_metric Y,
  apply_instance
end

/-- Given an embedding of a topological space into a metrizable space, the source space is also
metrizable. -/
lemma _root_.embedding.metrizable_space [metrizable_space Y] {f : X → Y} (hf : embedding f) :
  metrizable_space X :=
begin
  letI : metric_space Y := metrizable_space_metric Y,
  exact ⟨⟨hf.comap_metric_space f, rfl⟩⟩
end

instance metrizable_space.subtype [metrizable_space X] (s : set X) : metrizable_space s :=
embedding_subtype_coe.metrizable_space

instance metrizable_space_pi [Π i, metrizable_space (π i)] : metrizable_space (Π i, π i) :=
by { casesI nonempty_fintype ι, letI := λ i, metrizable_space_metric (π i), apply_instance }

variables (X) [t3_space X] [second_countable_topology X]

/-- A T₃ topological space with second countable topology can be embedded into `l^∞ = ℕ →ᵇ ℝ`.
-/
lemma exists_embedding_l_infty : ∃ f : X → (ℕ →ᵇ ℝ), embedding f :=
begin
  haveI : normal_space X := normal_space_of_t3_second_countable X,
  -- Choose a countable basis, and consider the set `s` of pairs of set `(U, V)` such that `U ∈ B`,
  -- `V ∈ B`, and `closure U ⊆ V`.
  rcases exists_countable_basis X with ⟨B, hBc, -, hB⟩,
  set s : set (set X × set X) := {UV ∈ B ×ˢ B| closure UV.1 ⊆ UV.2},
  -- `s` is a countable set.
  haveI : encodable s := ((hBc.prod hBc).mono (inter_subset_left _ _)).to_encodable,
  -- We don't have the space of bounded (possibly discontinuous) functions, so we equip `s`
  -- with the discrete topology and deal with `s →ᵇ ℝ` instead.
  letI : topological_space s := ⊥, haveI : discrete_topology s := ⟨rfl⟩,
  suffices : ∃ f : X → (s →ᵇ ℝ), embedding f,
  { rcases this with ⟨f, hf⟩,
    exact ⟨λ x, (f x).extend (encodable.encode' s) 0, (bounded_continuous_function.isometry_extend
      (encodable.encode' s) (0 : ℕ →ᵇ ℝ)).embedding.comp hf⟩ },
  have hd : ∀ UV : s, disjoint (closure UV.1.1) (UV.1.2ᶜ) :=
    λ UV, disjoint_compl_right.mono_right (compl_subset_compl.2 UV.2.2),
  -- Choose a sequence of `εₙ > 0`, `n : s`, that is bounded above by `1` and tends to zero
  -- along the `cofinite` filter.
  obtain ⟨ε, ε01, hε⟩ : ∃ ε : s → ℝ, (∀ UV, ε UV ∈ Ioc (0 : ℝ) 1) ∧ tendsto ε cofinite (𝓝 0),
  { rcases pos_sum_of_encodable zero_lt_one s with ⟨ε, ε0, c, hεc, hc1⟩,
    refine ⟨ε, λ UV, ⟨ε0 UV, _⟩, hεc.summable.tendsto_cofinite_zero⟩,
    exact (le_has_sum hεc UV $ λ _ _, (ε0 _).le).trans hc1 },
  /- For each `UV = (U, V) ∈ s` we use Urysohn's lemma to choose a function `f UV` that is equal to
  zero on `U` and is equal to `ε UV` on the complement to `V`. -/
  have : ∀ UV : s, ∃ f : C(X, ℝ), eq_on f 0 UV.1.1 ∧ eq_on f (λ _, ε UV) UV.1.2ᶜ ∧
    ∀ x, f x ∈ Icc 0 (ε UV),
  { intro UV,
    rcases exists_continuous_zero_one_of_closed is_closed_closure
      (hB.is_open UV.2.1.2).is_closed_compl (hd UV) with ⟨f, hf₀, hf₁, hf01⟩,
    exact ⟨ε UV • f, λ x hx, by simp [hf₀ (subset_closure hx)], λ x hx, by simp [hf₁ hx],
      λ x, ⟨mul_nonneg (ε01 _).1.le (hf01 _).1, mul_le_of_le_one_right (ε01 _).1.le (hf01 _).2⟩⟩ },
  choose f hf0 hfε hf0ε,
  have hf01 : ∀ UV x, f UV x ∈ Icc (0 : ℝ) 1,
    from λ UV x, Icc_subset_Icc_right (ε01 _).2 (hf0ε _ _),
  /- The embedding is given by `F x UV = f UV x`. -/
  set F : X → s →ᵇ ℝ := λ x, ⟨⟨λ UV, f UV x, continuous_of_discrete_topology⟩, 1, λ UV₁ UV₂,
    real.dist_le_of_mem_Icc_01 (hf01 _ _) (hf01 _ _)⟩,
  have hF : ∀ x UV, F x UV = f UV x := λ _ _, rfl,
  refine ⟨F, embedding.mk' _ (λ x y hxy, _) (λ x, le_antisymm _ _)⟩,
  { /- First we prove that `F` is injective. Indeed, if `F x = F y` and `x ≠ y`, then we can find
    `(U, V) ∈ s` such that `x ∈ U` and `y ∉ V`, hence `F x UV = 0 ≠ ε UV = F y UV`. -/
    refine not_not.1 (λ Hne, _), -- `by_contra Hne` timeouts
    rcases hB.mem_nhds_iff.1 (is_open_ne.mem_nhds Hne) with ⟨V, hVB, hxV, hVy⟩,
    rcases hB.exists_closure_subset (hB.mem_nhds hVB hxV) with ⟨U, hUB, hxU, hUV⟩,
    set UV : ↥s := ⟨(U, V), ⟨hUB, hVB⟩, hUV⟩,
    apply (ε01 UV).1.ne,
    calc (0 : ℝ) = F x UV : (hf0 UV hxU).symm
             ... = F y UV : by rw hxy
             ... = ε UV   : hfε UV (λ h : y ∈ V, hVy h rfl) },
  { /- Now we prove that each neighborhood `V` of `x : X` include a preimage of a neighborhood of
    `F x` under `F`. Without loss of generality, `V` belongs to `B`. Choose `U ∈ B` such that
    `x ∈ V` and `closure V ⊆ U`. Then the preimage of the `(ε (U, V))`-neighborhood of `F x`
    is included by `V`. -/
    refine ((nhds_basis_ball.comap _).le_basis_iff hB.nhds_has_basis).2 _,
    rintro V ⟨hVB, hxV⟩,
    rcases hB.exists_closure_subset (hB.mem_nhds hVB hxV) with ⟨U, hUB, hxU, hUV⟩,
    set UV : ↥s := ⟨(U, V), ⟨hUB, hVB⟩, hUV⟩,
    refine ⟨ε UV, (ε01 UV).1, λ y (hy : dist (F y) (F x) < ε UV), _⟩,
    replace hy : dist (F y UV) (F x UV) < ε UV,
      from (bounded_continuous_function.dist_coe_le_dist _).trans_lt hy,
    contrapose! hy,
    rw [hF, hF, hfε UV hy, hf0 UV hxU, pi.zero_apply, dist_zero_right],
    exact le_abs_self _ },
  { /- Finally, we prove that `F` is continuous. Given `δ > 0`, consider the set `T` of `(U, V) ∈ s`
    such that `ε (U, V) ≥ δ`. Since `ε` tends to zero, `T` is finite. Since each `f` is continuous,
    we can choose a neighborhood such that `dist (F y (U, V)) (F x (U, V)) ≤ δ` for any
    `(U, V) ∈ T`. For `(U, V) ∉ T`, the same inequality is true because both `F y (U, V)` and
    `F x (U, V)` belong to the interval `[0, ε (U, V)]`. -/
    refine (nhds_basis_closed_ball.comap _).ge_iff.2 (λ δ δ0, _),
    have h_fin : {UV : s | δ ≤ ε UV}.finite, by simpa only [← not_lt] using hε (gt_mem_nhds δ0),
    have : ∀ᶠ y in 𝓝 x, ∀ UV, δ ≤ ε UV → dist (F y UV) (F x UV) ≤ δ,
    { refine (eventually_all_finite h_fin).2 (λ UV hUV, _),
      exact (f UV).continuous.tendsto x (closed_ball_mem_nhds _ δ0) },
    refine this.mono (λ y hy, (bounded_continuous_function.dist_le δ0.le).2 $ λ UV, _),
    cases le_total δ (ε UV) with hle hle,
    exacts [hy _ hle, (real.dist_le_of_mem_Icc (hf0ε _ _) (hf0ε _ _)).trans (by rwa sub_zero)] }
end

/-- *Urysohn's metrization theorem* (Tychonoff's version): a T₃ topological space with second
countable topology `X` is metrizable, i.e., there exists a metric space structure that generates the
same topology. -/
lemma metrizable_space_of_t3_second_countable : metrizable_space X :=
let ⟨f, hf⟩ := exists_embedding_l_infty X in hf.metrizable_space

instance : metrizable_space ennreal := metrizable_space_of_t3_second_countable ennreal

end topological_space